1. Field of the Invention
This invention relates to a document feeding device for automatically feeding document sheets one by one to an image reading portion of an image processing system such as a copying machine, and more particularly to an automatic document feeding device capable of automatically feeding large-size document sheets by use of a compact document feeder unit designed for feeding document sheets of relatively small size. 2. Description of the Prior Art
In image processing systems such as copying machines and image scanners, document feeding means of various types are used for feeding document sheets to the image reading portion prescribed in the respective systems. A tendency to employ, as office machines, copying machines capable of dealing with as document sheets large as or larger than A3 size (297 mm.times.420 mm) is seen lately.
In an ordinary copying machine, a document sheet to be read is fed to a reading portion defined on a transparent platen for copying, and then, discharged from the reading portion after copying by operating a document feeder unit. That is, the document sheet is moved over the platen until the leading end thereof reaches the reference point for image reading (exposure standard point). The document feeder unit for conveying the document sheet generally comprises rotating endless belt means made of flexible material such as rubber lying over the platen.
The platen in the conventional copying machine has the effective copying area equal to or somewhat larger than the document sheet to be dealt with by the copying machine. Therefore, the document feeder unit has a plane area substantially equal to the platen, namely, has had to be made larger than the document sheet to be copied, resulting in a large overall size and heavy weight. Usually, the feeder unit can be opened over the platen so as to manually place the document sheet on the reading portion defined on the platen. However, this would turn out to a troublesome chore.
Furthermore, it is not rational to automatically feed small-size document sheets by using the document feeder unit which is designed for dealing with large-size document sheets. In general, a document sheet of relatively small size (e.g. A4 size and ledger size) is very frequently utilized. The conventional document feeder unit has suffered a disadvantage that the small-size document sheet is dealt with similarly to the document sheet of large size (e.g. A3 size). It therefore would be ideal that a compact document feeder unit which is designed primarily for automatically feeding small-size document sheets can take care of the large-size document sheets.
For the purpose of rationally dealing with document sheets of different sizes, there have been proposed document feeding devices in U.S. Pat. No. 4,922,292 (corresp. to Japanese Patent Appln. Public Discl. HEI 2(1990)-13532(A)). and Japanese Patent Appln. Public Disclosures HEI 2(1990)-13534(A) and HEI 2(1990)-13579(A). These conventional document feeding devices all have a semi-automatic document feeder which can automatically feed only document sheets of relatively small size e.g. A4 size). That is, though the aforementioned conventional document feeder makes it possible to automatically feed the small-size document sheet to a prescribed location in a copying machine or the like, document sheets of large size (e.g. A3 size) cannot be automatically fed and therefore must be manually positioned at the prescribed location by opening the the document feeder. The conventional document feeder can avoid the increase in size, but not satisfy the requirements for full automation and rationality of feeding document sheets of different sizes in a copying machine and so on.